The invention relates to a microporous film that contains at least polyethylene having an intrinsic viscosity of more than 5 dl/g (measured at 135xc2x0 C. in decalin) and a porosity of at most 70 vol. %. In particular the invention relates to a microporous film that is suitable for use as a battery separator.
Such a microporous film is known from JP-A-08034873. In Experiment 27, JP-A-08034873 describes a microporous film that contains polyethylene having an intrinsic viscosity of 14 dl/g and a porosity of 52.5 vol. %. A film suitable for use as a battery separator is preferably as thin as possible. For this reason the porosity is at most 70 vol. %. The film""s weight per unit area and per unit thickness is preferably as large as possible and the film preferably has the highest possible puncture resistance, to prevent damage by sharp parts of the electrodes during the winding of the film. A film suitable for use as a battery separator also has the highest possible conductance. As the conductance depends on the number of open pores, the air permeability, expressed in the Gurley value, is a measure of conductance. The Gurley value is determined in s/50 ml according to ASTM standard D 726, using a measuring area of 6.45 cm2 (1 square inch) and a weight of 567 grams. A low Gurley value means that the film has a high air permeability and hence also high conductance.
The aim of the invention is to provide a microporous film, which, in the combination of properties mentioned above, is more suitable for use as a battery separator than the known films.
This aim is achieved in that the film has a battery separator""s quality factor (F) that is at least 2.5 and
F=BWxc2x7PR/(Gxc2x7t), 
where BW is the basic weight (g/m2), PR the puncture resistance in g, G the Gurley in sec/50 ml and t the film""s thickness in xcexcm.
This makes the combination of desired properties for a film intended for use as a battery separator superior to that of the known films.
xe2x80x98Intrinsic viscosityxe2x80x99 is in this text understood to be the intrinsic viscosity measured according to ASTM D 4020, in decaline at 135xc2x0 C. (dl/g).
xe2x80x98Puncture resistancexe2x80x99 is in this text understood to be the puncture resistance measured according to DIN 53373(g).
The xe2x80x98Gurleyxe2x80x99 is in this text understood to be the Gurley measured according to ASTM D 726 (sec/50 ml). The film""s thickness (t) is the thickness measured according to ISO 4593 (xcexcm).
The invention also relates to a process for preparing a microporous film according to the invention. A known process for preparing a microporous film is described in JP-B-8-34873. This describes a process for preparing a microporous polyethylene film by forming a homogeneous polyethylene solution having an intrinsic viscosity of more than 5 dl/g into a film, cooling the film and biaxially stretching the cooled film.
A drawback of this process is that the known process described in JP-A-08034873 cannot be used to produce films having a battery separating quality factor of 2.5 of more.
The aim of the invention is to provide a process by which a microporous film with a battery separating quality factor of at least 2.5 can be produced.